The invention relates to a process for the heat treatment of structure castings made from an aluminum alloy and to an aluminum alloy to be used for this purpose.
Aluminum structure castings made from an aluminum alloy are used, for example, in the automotive industry and should have good mechanical properties, in particular a high elongation at break, good castability, no tendency to stick in the mold and good mold-release properties, a high design strength and good-weldability. Since the known aluminum casting alloys-do not have the required properties in the cast state, heat treatment processes and aluminum alloys have been developed to enable industrial requirements to be satisfied to an ever more accurate and less expensive extent. Special heat treatment processes designated T64 and T7 have become known for this process. These heat treatment processes are described, for example, in xe2x80x9cDas Techniker Handbuchxe2x80x9d [The Engineering Handbook] Bxc3x6ge, Vieweg, 13th Edition, pages 551 to 554. These heat treatment processes involve a two-stage procedure as detailed below:
T64 (Thermally Unstable):
1st stage: Heating to 480 to 520xc2x0 C., holding for 2 to 5 hours, quenching in water at 20xc2x0 C.;
2nd stage: Heating to 155 to 170xc2x0 C., holding for 2 to 6 hours, quenching in air.
T7 (Thermally Stable up to 230xc2x0 C.):
1st stage: Heating to 480 to 520xc2x0 C., holding for 2 to 5 hours, quenching in water at 20xc2x0 C.
2nd stage: Heating to 200 to 230xc2x0 C., holding for 2 to 3 hours, quenching in air.
The structure castings which have been treated using the heat treatment process T64 are not thermally stable at elevated temperatures, but castings which have been treated using heat treatment process T7 are stable at elevated temperatures. A drawback of both heat treatment processes T64 and T7 is that the structure castings produced by means of the die-casting process lose their extremely high dimensional accuracy which is present in the cast state, on account of the high thermal stress states which occur in the structure casting during the quenching in water. The structure castings are dimensionally unstable after the first heat treatment stage and have to be dimensionally accurate by expensive and complicated straightening operations. This problem is particularly acute in structure components, since these structure castings have a high level of complexity and integrity and have to satisfy high demands imposed on the dimensional accuracy.